Not applicable
This invention relates in general to steam generators and more particularly to evaporator for a steam generator and to an evaporation process.
Much of the equipment for generating electrical power relies on steam, and so do a variety of industrial processes. In either case, hot gases, in many instances generated by combustion, pass through a generator which converts water into superheated steam. Typical of these installations are heat recovery steam generators (HRSGs) which are used to extract heat from the hot gases discharged by gas turbines that drive electrical generators. The heat extracted produces steam which passes on to a steam turbine that powers another electrical generator.
The typical steam generator, aside from a duct through which the hot gases pass, in its most basic form, includes three additional componentsxe2x80x94namely, a superheater, an evaporator, and an economizer or feedwater heater arranged in that order with respect to the flow of gases in the duct. The water flows in the opposite direction, that is through the economizer where it is heated, but remains a liquid, then through the evaporator where it is converted into saturated steam, and then through the superheater where the saturated steam becomes superheated steam.
Evaporators come in two basic configurationsxe2x80x94the circulation type and the once-through typexe2x80x94each with its own advantages and disadvantages. Both have an array of tubes in the duct through which the hot gases pass.
In the circulation type, the tubes reside in a circuit with a steam drum that is above the tubes. The drum contains water which flows from the drum, through a downcomer, and then into the tubes where some of it is converted into steam, but the steam exists as bubbles within the water, and is returned through a riser into the steam drum. Here the steam, which is saturated, separates from the liquid water and passes onto the superheater. It is replaced by feedwater which is supplied to the drum. The tubes of a circulation evaporator remain wet all the timexe2x80x94that is to say, liquid water exists against their interior surfaces throughoutxe2x80x94and this promotes good heat transfer. Moreover, impurities, such as dissolved salts, concentrate in the water within the drum and the remainder of the circulation loop, leaving the saturated steam that escapes largely free of them. A small water flow, known as blowdown, is extracted from the drum to control the accumulation of impurities. Most circulation evaporators rely entirely on the variance in density between the water in the downcomer and the water-steam mixture in the tubes to circulate the water in the evaporator, although some have a pump assist. Furthermore, a circulation evaporator contains a reservoir of stored water. Thus, the failure of a pump does not immediately affect the operation of the evaporator and render it vulnerable to overheating. Also, circulation evaporators operate very well over a wide range of load conditions. Finally, circulation evaporators predominate, and as a consequence boiler operators are familiar with their operation.
But circulation evaporators have their detractions. Perhaps the greatest of these is the expense attributable to steam drums, large downcomers and headers to supply water to their tubes. Moreover, the reservoirs of water contained in them require time to bring up the boiling temperature, so the start-up time for a circulation evaporator is extended.
Once-through evaporators do not require downcomers or drums, so the only stored water in them resides in the tubes themselves. This enables a once-through evaporator to be brought to operating conditions more rapidly than a natural circulation evaporator. However, a once-through evaporator must completely convert the water into steam, so that only saturated steam escapes and flows on to the superheater. No liquid water should leave the evaporator. As a consequence, regions of the tubes run dry, that is to say, their interiors are not wetted by liquid water. The transfer of heat diminishes significantly in these regions, even though the regions operate at temperatures in excess of the wetted regions. Some manufactures of once-through evaporators resort to high alloy metals to enable the tubes to better withstand the elevated temperatures. Whereas, a circulation evaporator discharges steam that is largely free of impurities, a once-through evaporator will discharge steam containing all the impurities present in the feedwater that is pumped into it. Therefore, the feedwater needs to be treated to eliminate as many impurities as possible.
Thus, circulation and once-though evaporators each have advantages and disadvantages.
The present invention resides in an evaporator that possesses many of the advantages of both a circulation evaporator and a once-through evaporator, but few of the disadvantages. To this end, it includes first tubes located in a flow of hot gasses, second tubes also located in the flow, and a vessel connected to both the first and second tubes such that it receives water from the first tubes and such that water from in the vessel circulates through the second tubes and back to the vessel. The invention also resides in the process embodied in the operation of the evaporator.